Inbound interference reduction in a broadband powerline system

ABSTRACT

Disclosed is a method and apparatus for reducing inbound interference in a broadband powerline communication system. Data modulated on first and second carrier frequencies is received via respective first and second lines of the powerline system. A characteristic of at least one of the carrier signals (e.g., phase or amplitude) is adjusted at the receiver in order to reduce the effects of inbound interference on the transmission system. The adjustment parameters may be determined by adjusting the parameters, during a period of no data transmission, until the output of a differential receiver is zero.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior U.S. patent application Ser.No. 11/417,435, filed May 3, 2006 and issued as U.S. Pat. No. 7,453,353on Nov. 18, 2008, which is a continuation of U.S. patent applicationSer. No. 10/840,096, filed May 6, 2004 and issued as U.S. Pat. No.7,091,849 on Aug. 15, 2006, all of which are hereby incorporated hereinby reference.

This application is related to commonly assigned patent application Ser.No. 10/839,945 filed May 6, 2004 entitled Outbound InterferenceReduction in a Broadband Powerline System, which is incorporated hereinby reference.

BACKGROUND OF THE INVENTION

This application relates generally to data transmission, and moreparticularly to data transmission over power lines.

The use of power lines to transmit data is known. Initially, powerlinecommunication systems were limited to relatively low data rates,typically less than 500 kbs. These low data rates are generally usefulfor applications such as remote control of various switches connected tothe powerline system. More recently, developments have been made in thearea of broadband powerline communication systems, also known aspowerline telecommunications (PLT) systems or broadband powerline (BPL)systems. These systems are capable of transmitting data at significantlyhigher data rates than previous systems. For example, BPL systems cantransmit data at rates of 4-20 Mbps.

While existing powerline systems are capable of transmitting data at therates described above, they were not initially designed for datatransmission. Instead, they were designed to carry large currents athigh voltages so that significant amounts of energy could be distributedat one primary low frequency (e.g., 60 Hertz).

Powerline communication systems generally use one or more carrierfrequencies in order to spread the data transmission over a wider rangeof frequencies. The low data rate powerline communication systemsdiscussed above generally utilized frequencies in the range of 9 kHz to525 kHz. In these low data rate systems, the risk of interference fromexternal radiation sources is low. The high data rates of BPL systemscannot be achieved using carrier frequencies below 525 kHz. Instead, BPLsystems typically use carrier frequencies in the range of 1-30 MHz.

One of the problems with a BPL system is the detrimental effects ofinbound interference from external electromagnetic radiators. Thephysical attributes of the power lines (e.g., high elevation andunshielded wiring) along with the higher carrier signal frequenciesneeded for high bandwidth data transmission, contribute to thisinterference problem.

BRIEF SUMMARY OF THE INVENTION

I have recognized that a power line acts as an antenna and may bemodeled using antenna analysis techniques. This recognition has led toadvantageous techniques for reducing the effects of inbound interferencein a powerline communication system.

In accordance with one embodiment of the invention, signals are receivedon first and second lines of the powerline system. The signals comprisea modulated carrier signal component and an interference component. Atleast one characteristic of at least one of the first and second signalsis adjusted in order to reduce the effects of inbound interference inthe powerline system. The adjusted characteristic may be, for example,signal phase or signal amplitude.

In accordance with another embodiment of the invention, the powerlinecommunication system is a frequency division multiplexed systemtransmitting data on a plurality of frequency channels and the signalcharacteristics are adjusted independently for each of the frequencychannels.

In accordance with one embodiment of the invention, the adjustmentsettings are determined by adjusting the characteristics until theoutput of a differential receiver operating on the carrier signals iszero while there is no data transmission taking place in the powerlinecommunication system.

These and other advantages of the invention will be apparent to those ofordinary skill in the art by reference to the following detaileddescription and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical prior art powerline communication system; and

FIG. 2 shows a powerline communication system incorporating anembodiment of the invention.

DETAILED DESCRIPTION

A typical prior art powerline communication system 100 is shown inFIG. 1. A head end network node 106 is connected to a data network 102via a fiber optic cable 104. In accordance with a typical networkservice, the head end 106 is configured to transmit data to end userpremises (e.g., premises 108) using powerline cables as the transmissionmedium. The head end 106 is also configured to convert signals in theoptical, domain received from fiber 104 to the electrical domain usingwell known optical to electrical conversion techniques. The head end 106is connected to a transmitter 110. The transmitter 110 contains amodulator 112 which modulates the data received from head end 106 onto acarrier signal using well known RF modulation techniques. As describedabove, typical carrier frequencies for a powerline communication systemare in the range of 2-30 MHz. The modulated signal is provided to thepowerline cable 114 via line 116 and coupler 118. A powerlinecommunication system 100 of the type shown in FIG. 1 may use orthogonalfrequency division multiplexing (OFDM) in which the available bandwidthis split up into multiple narrowband channels which do not interferewith each other. Thus, in accordance with OFDM transmission, multiplecarrier signals, each having its own frequency band and representing adistinct data channel, are carried over the cable 114.

For purposes of the present description, it is assumed that thepowerline cable 114 is a medium voltage (MV) powerline cable typicallysupplying power at 4-66 kV. Such medium voltage cable is typically analuminum cable having a 1 cm diameter. Coupler 118 couples the modulatedcarrier signal supplied by line 116 to the MV line 114. Various types ofcouplers 118 are known in the art. For example, coupler 118 may be aninductive coupler, a capacitive coupler, or may employ direct metalliccontact. The carrier signal is transmitted along the length of MVpowerline cable 114 to coupler 120 which couples the signal from the MVpowerline cable 114 to a receiver 124 via line 122.

The signal from receiver 124 is provided to the premises 108 via lowvoltage (LV) powerline 128. The low voltage powerline typically supplypower at 100-240 volts. Thus, one of the functions of the receiver is totranslate the data from the MV line to the LV line. The low voltage lineis connected to a modem 130 within the premises 108. The modem 130demodulates the signal received from the MV powerline cable 114 andextracts the data that was transmitted from the head end 106. It isnoted that in particular embodiments, it is possible that the receiver124 further functions to demodulate the data and deliver it to a secondtransmitter (not shown) that would re-modulate the data and send it tothe premises 108.

It is noted that for ease of description only downstream (i.e., fromhead end to end user) data transmission is shown and described. Oneskilled in the art would readily recognize that upstream transmissioncould be accomplished in a similar manner.

As described above in the background section, one of the problems withpowerline data transmission systems as shown in FIG. 1 is the effect ofinbound interference. As such, the signal received by a receiver in apowerline system comprises a modulated carrier signal component and aninterference component.

I have recognized that a MV powerline acts as an antenna and may bemodeled using antenna analysis techniques. Using the assumptionsdescribed above, and depending upon the effective terminating impedancepresented by the couplers, the MV line may be considered to be a dipoleantenna (approximately several wavelengths long) or a traveling-wave(Beverage) antenna. In either case, the power line's ohmic resistance isless than 2 ohms, and so dissipation is negligible. The powerline wireradiates approximately half the power launched in each direction andmakes the remaining half available at the termination points. For eitherthe dipole or the traveling-wave antenna, the effective gain G of thewire is approximately 0-10 dB, depending upon the wavelength.

If P is the power launched onto the wire, then the Effective IsotropicRadiated Power (EIRP) is defined as

${E\; I\; R\; P} \approx {\left( \frac{P}{2} \right)G}$

In the United States, Part 15 of the Federal Communications CommissionRules, (47 CFR 15) sets forth the regulations under which anintentional, unintentional, or incidental radiator may be operatedwithout an individual license. Under these rules, the upper limit onallowable launched power is give by:

$\frac{E\; I\; R\; P}{4\;\pi\; r^{2}} < \frac{E\;\max^{2}}{Zfs}$where r=30 m, Emax=30 uV/m in 9 KHz and Zfs=377 ohms. For G=10, thisputs an upper limit on launched power of Pmax=−52 dBm in a 9 KHzchannel. See, e.g., 47 CFR 15.109, 15.209.

The lower limit on launched power is set by the interferenceenvironment. Assume, for example, that we want to protect againstincoming interference with a margin of 10 dB. For strong interference,e.g., received level of S9 or −73 dBm, desired signal power at thereceiver must be greater than −73 dBm+10 dB or −63 dBm, so the launchedpower must be greater than −60 dBm. (Since only about half of thelaunched power is available at the receiver). Thus, the launched power(in a 9 KHz slot) is bounded by:−60 dBm<launched power<−52 dBm.

The above described model defines the basic constraint on the signalpower levels in a BPL system. For reasonable system parameters, there isan operating window, within which it is possible to simultaneouslysatisfy the FCC requirements and also provide some margin againstoutside interference.

As described in co-pending patent application Ser No. 10/839,945 filedMay 6, 2004, the key to reducing outbound interference effects of a BPLsystem is to reduce the gain G of the power lines which are acting as anantenna. Such a reduction in gain G has several benefits. For example,if G is reduced by 10 dB, then the signal power required at the receiverto maintain margin against a given outside interferer is reduced by alike amount, and thus radiated interference is reduced by 20 dB. Theabove referenced related patent application describes that anadvantageous technique for reducing G is to use a balanced transmissionline, which may be achieved by using two wires and differentialexcitation. Balanced data transmission is well known in the art of datatransmission, and generally requires at least two conductors per signal.The transmitted signal is referenced by the difference of potentialbetween the lines, not with respect to ground. Thus, differential datatransmission reduces the effects of noise, which is seen as common modevoltage (i.e., seen on both lines), not differential, and is rejected bydifferential receivers. In the simplest type of differential datatransmission system, the same signal is transmitted via bothtransmission lines, with the phase of the signals being offset form eachother by 180 degrees. More sophisticated differential systems allow forthe adjustment of the relative phase and the amplitude of the twotransmitted signals. For an ideal balanced line, G=0 and there is nointerference. For two parallel wires separated by a non-infinitesimaldistance d, the field strength at a distance r is reduced byapproximately d/r compared with the single-wire case. Thus for d=1 m andr=30 m, G is reduced by approximately 30 dB.

The realization that the powerline system may be modeled using antennaanalysis techniques has led to the further realization that the effectsof inbound interference in a powerline communication system may bereduced by applying signal adjustments at an adjustment module of adifferential receiver.

A first embodiment of the present invention is shown in FIG. 2. FIG. 2shows a powerline communication system 200 comprising a transmitter 202coupled to a first powerline cable 210 and a second powerline cable 212via couplers 214 and 216 respectively. As described in conjunction withtransmitter 110 of FIG. 1, transmitter 202 encodes data received from anetwork node (e.g., a head end 106 as shown in FIG. 1) for transmissionvia the power lines. The transmitter 202 contains a modulator 204 formodulating a carrier signal with the data to be transmitted using wellknown modulation techniques. The embodiment shown in FIG. 2 usesdifferential data transmission whereby a first carrier signal ismodulated and coupled to power line 210 via coupler 214 and a secondcarrier signal is modulated and coupled to power line 212 via coupler216. The signals are received via couplers 218 and 220 which areconnected to a differential receiver 222. Differential receiver 222responds to the difference between the signals receive via coupler 218and 220, and transmits the difference signal to a modem 224 within thepremises 226. The modem 224 demodulates the signal received from the MVpower lines to extract the transmitted data.

In accordance with known differential data transmission techniques, bothcarrier signals have the same frequency and are modulated with the samedata, but the carrier signals are transmitted having different phases.In accordance with known differential data transmission techniques, thecarrier signals would be out of phase with each other by 180 degrees.

Due to the possibility of external interference, the signal received atdifferential receiver 222 may be degraded due to such interference. Assuch, the signals receive via couplers 218 contain a modulated carriersignal component as well as an interference component. In accordancewith one embodiment of the invention, an adjustment module 228 is usedin connection with differential receiver 222. The adjustment module 228is configured to adjust the characteristics of the signal received online 230 in order to remove or reduce the overall inbound interferenceeffects. Such characteristics may be, for example, the phase oramplitude of the received signal. In accordance with one embodiment, theadjustment module is configured as follows. First, all transmitters inthe system stop transmitting. Therefore, any signal received atdifferential receiver 222 will be the result of an external interferencesource. The adjustment module 228 then adjusts at least onecharacteristic of the signal on line 230 in order to produce a zerooutput from the differential receiver 222. At the point where zerooutput is received from the differential receiver, the adjustmentparameters of the adjustment module are recorded. The adjustmentparameters may be, for example, the phase and/or amplitude adjustmentbeing applied by the adjustment module. These adjustment parameters arethe parameters required in order to remove the effect of the externalsource of interference. These same adjustment parameters are then usedduring data transmission, such that the adjustment parameters willcontinue to remove the effects of the external interference source. Oneskilled in the art would readily recognize that the adjustment module228 could adjust the characteristics of the carrier signal received online 232 instead of line 230. In yet other embodiments, the adjustmentmodule 228 could be configured to adjust the characteristics of thecarrier signals received on both lines 230 and 232.

The adjustment parameters may also be determined while data is beingtransmitted if the characteristics of the interference are known (i.e.,the interference has a known signature). In such a case, the adjustmentmodule adjusts at least one characteristic of the received signal untilthe interference is removed. At the point where the known interferencesignal is removed, the adjustment parameters of the adjustment moduleare recorded and used as described above.

The embodiment shown in FIG. 2 is particularly advantageous whenOFDM-data transmission is utilized, because each frequency channel maybe individually adjusted in order to remove the external interferencesource. In such an embodiment, the adjustment module 228 adjusts thesignal characteristics of each narrowband carrier signal individually,because the external interference may affect different frequencychannels in the OFDM system differently.

The above described embodiment assumes that the external interferencesource is constant over some time period. In various embodiments, thetransmitters of the powerline communication system could stoptransmitting periodically in order to allow the adjustment module 228 toreadjust its settings in order to deal with varying interference sourcesover time. When the transmitters stop transmitting, the adjustmentmodule 228 performs the above described steps for setting its adjustmentparameters.

In another possible embodiment of the invention, the differentialreceiver could be manually preconfigured in order to deal with a knownexternal interference source. One skilled in the art would recognizethat many variations are possible. For example, if the interferingsignal source has known signature, then the adjustment module could beconfigured to reduce interference based on the signature. For example,the interfering source may only transmit periodically at known times, inwhich case the adjustment module would be configured to adjust theincoming signal appropriately only when the interfering signal source istransmitting. In yet another alternative, the signature could bedetermined dynamically as well.

The foregoing Detailed Description is to be understood as being in everyrespect illustrative and exemplary, but not restrictive, and the scopeof the invention disclosed herein is not to be determined from theDetailed Description, but rather from the claims as interpretedaccording to the full breadth permitted by the patent laws. It is to beunderstood that the embodiments shown and described herein are onlyillustrative of the principles of the present invention and that variousmodifications may be implemented by those skilled in the art withoutdeparting from the scope and spirit of the invention.

1. A method for transmitting data comprising: determining an adjustmentparameter; and adjusting by the adjustment parameter a characteristic ofa first signal transmitted on a first line of a powerline transmissionsystem and a characteristic of a second signal transmitted on a secondline of the powerline transmission system; wherein determining theadjustment parameter comprises adjusting the characteristic of at leastone of the first signal and the second signal until an output of adifferential receiver operating on the first signal and the secondsignal is zero.
 2. The method of claim 1 further comprising: receivingthe first signal, the first signal comprising a first modulated carriersignal component and a first interference component; and receiving thesecond signal, the second signal comprising a second modulated carriersignal component and a second interference component.
 3. The method ofclaim 2 wherein the first modulated carrier signal component is 180degrees out of phase with the second modulated carrier signal component.4. The method of claim 1 wherein determining the adjustment parameter isperformed during a time period when data is not being transmitted viathe powerline transmission system.
 5. The method of claim 1 whereindetermining the adjustment parameter is performed during a time periodwhen data is being transmitted via the powerline transmission system. 6.The method of claim 1 wherein the powerline transmission system is afrequency division multiplexed system transmitting data on a pluralityof frequency channels.
 7. The method of claim 6 wherein determining theadjustment parameter is performed independently on each of the pluralityof frequency channels.
 8. The method of claim 1 wherein the adjustmentparameter varies periodically.
 9. A differential receiver comprising: afirst receiver adapted to receive a first signal transmitted on a firstline of a transmission system, the first signal comprising a firstmodulated carrier signal component and a first interference component; asecond receiver adapted to receive a second signal transmitted on asecond line of the transmission system, the second signal comprising asecond modulated carrier signal component and a second interferencecomponent; and an adjustment module adapted to adjust a characteristicof the first signal and a characteristic of the second signal by theadjustment parameter, the adjustment module determining the adjustmentparameter by adjusting the characteristic of at least one of the firstsignal and the second signal until an output of the differentialreceiver operating on the first signal and the second signal is zero.10. The differential receiver of claim 9 wherein the transmission systemis adapted to transmit the first modulated carrier signal component 180degrees out of phase from the second modulated carrier signal component.11. The differential receiver of claim 9 wherein the transmission systemis a frequency division multiplexed system adapted to transmit data on aplurality of frequency channels.
 12. The differential receiver of claim9 wherein the adjustment module is adapted to adjust the characteristicof the first signal and the characteristic of the second signal toreduce an effect of external interference.
 13. The differential receiverof claim 9 wherein the adjustment module is adapted to vary theadjustment parameter periodically.
 14. The differential receiver ofclaim 9 wherein the differential receiver is coupled to a modem.
 15. Thedifferential receiver of claim 9 wherein the transmission system is apowerline transmission system.
 16. A powerline communication systemcomprising: a differential receiver coupled to a first transmission lineand a second transmission line, the differential receiver adapted toreceive a first modulated carrier signal and a second modulated carriersignal via the first and second transmission lines respectively; and anadjustment module adapted to adjust a characteristic of the firstmodulated carrier signal and a characteristic of the second modulatedcarrier signal by an adjustment parameter, the adjustment moduledetermining the adjustment parameter by adjusting the characteristic ofat least one of the first modulated carrier signal and the secondmodulated carrier signal until an output of the differential receiveroperating on the first modulated carrier signal and the second modulatedcarrier signal is zero.
 17. The powerline communication system of claim16 further comprising a modem adapted to demodulate the first modulatedcarrier signal and the second modulated carrier signal to extract data.18. The powerline communication system of claim 16 wherein theadjustment module is adapted to vary the adjustment parameterperiodically.
 19. The powerline communication system of claim 16 whereinthe adjustment module is adapted to adjust the characteristic of the atleast one of the first modulated carrier signal and the second modulatedcarrier signal by the adjustment parameter to reduce an effect ofexternal interference on the first modulated carrier signal and thesecond modulated carrier signal.